A biogas may be produced through the anaerobic digestion or fermentation of a material containing biomass. The feedstock is typically a waste product, such as manure or sewage. The biogas is typically comprised of 50-75% methane and 25-50% carbon dioxide. Other gases, such as nitrogen, hydrogen, hydrogen sulfide or oxygen may be also present but collectively are unlikely to account for more than 10% of the biogas. Of these other gases, nitrogen is likely to be the largest component. The biogas can be burned directly with oxygen and so is usable as a fuel. The methane within the biogas can also be concentrate to provide a replacement for natural gas.
Biogas can be produced in an anaerobic digester. The digestion process involves microorganisms, primarily bacteria, which break down or convert the input materials to produce the biogas and an effluent. The process involves a series of bacteria types and processes, primarily hydrolysis, acidogenesis, acetogenesis and methanogenesis. The effluent, which may be called digestate, contains residual solid and liquid products. Besides producing useful biogas, the solid component of digestate is reduced in volume and mass compared to the feedstock. Some of the products within the digestate are useful, for example as a fertilizer, but others are waste products or require further processing to produce useful products. Accordingly, the digestate must be separated, and some of its separated components further processed, to produce useful products or safely dispose of it.
The principal component of the digester is a sealed tank, which contains the feed undergoing digestion and the active bacteria. Depending on the species of methanogenic bacteria used, the reactor may be operated at mesophilic (20-45 C, preferably 37-41 C) or thermophilic (50-70 C, preferably 50-52 C) temperatures. The tank may be filled in batches or operated with continuous (including semi-continuous or periodic) feed, digestate and gas removal. The digester may operate in a high solids mode, with a total suspended solids (TSS) concentration of up to 20%, or in a low solids mode with TSS of 15% or less, more often 10% or less. Continuous reactor designs include continuously stirred-tank reactors (CSTR), upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB) and internal circulation reactors (IC), in single or multiple stages. Operation in low solids mode requires a larger reactor but has some advantages. For example, less energy is required to mix the contents of a low solids mode digester due to reduced viscosity. Further, the effluent from a low solids mode digester may be handled using ordinary pumps whereas high solids mode digestion produces a thick sludge effluent that is difficult to handle.
The composition of the feedstock is important to the biogas generation process. Anaerobic digesters were originally designed primarily for use with cattle manure and sewage. Other feedstock may have a different composition of biodegradable material. In general, simple carbohydrates are easiest to digest whereas large molecules are more difficult to digest. The carbon to nitrogen ratio of the feedstock is also relevant, with a C:N ratio of 20-30:1 being preferred. The moisture content or solids concentration may also vary between feedstocks.